1. Field of the Invention
This invention relates to a system and method for the cathodic protection of structures such as pipelines and well casings disposed in an electrically conducting medium such as the ground and more particularly to such a system utilizing pulsed D.C. current to protect a plurality of such structures in which the spacing between the structures and/or different electrical properties of the conducting medium surrounding the structures are not amenable to the use of a single pulsed source.
2. Description of the Prior Art
The use of cathodic protection to prevent corrosion is well established for the protection of metal structures, such as well casings and pipe lines, that are buried in conductive soils. Cathodic protection is also used for the protection of inner surfaces of tanks which contain corrosive solutions, as well as for the protection of sub-platforms, and other offshore metal structures. It is well established that the cathodic protection can be accomplished either by the use of sacrificial anodes electrically grounded to the structure to be protected, or by the application of low voltage direct current from a power source. In the latter method steady direct current, half or full wave rectified current, and pulsed direct current have all been used.
It has been well established that, when a cathodic protection current is applied to a circuit including the structure (cathode) to be protected and its associated anode, a layer of charge is formed at approximately 100 A. from the surface of the structure. This layer of charge is called a taffel double layer. This layer acts as a capacitor in series with the anode-cathode circuit. In the absence of a cathodic protection system the soil or other conductive corrosive medium to which a ferrous metal structure such as a steel pipeline is exposed will cause an adverse chemical reaction in which ferrous or iron molecules pass into solution as positive ions by surrendering electrons to the structure. Hydrogen ions in the solution will accept the free electrons and form a gas, e.g. H.sub.2, adjacent to the surface of the structure. Oxygen molecules and certain other substances, if present in the solution, will also accept the electrons. This action results in a loss of iron in the structure with a consequent degradation of structural integrity.
Direct current cathodic protection systems prevent (or inhibit) the iron molecules from passing into solution by providing an exterior source of free electrons to the structure. The electrons supplied by the cathodic protection systems reduce any oxygen molecules and/or hydrogen ions present at the surface of the structure. The iron molecules are inhibited from going into solution, because the hydrogen ion and oxygen molecule receptors for the iron molecule electrons have been reduced by the cathodic protection system electrons. As a general rule, the greater the amount of current (accumulated electrons per unit of time) that is supplied by the cathodic protection system, the greater will be the area of structure protected.
A typical steady state 15 volt and 15 ampere D.C. cathodic protection system offers good protection but provides only a limited umbrella of protection or throw along the structure such as a pipeline to be protected. Such steady state systems thus require a considerable number of protection stations for a given length of the structure or pipe to be protected. Increasing the amount of current supplied by increasing the voltage, will increase the throw. The average current must, however, be limited such that an excess of hydrogen gas is not generated at the point of application of the cathodic protection system. An excess of hydrogen may cause damage to protective coatings. Excess hydrogen will also permeate the pipe wall, causing certain pipe materials to crack or rupture.
It has been shown that a pulsed D.C. voltage source having an output of the order of 100-300 volts for 5-100 microseconds (".mu.s") with a duty cycle of the order of 10% provides a much greater coverage (or throw) per station e.g. one station every few miles of pipeline. Such pulsed systems have been considered to be particularly effective because, although the average current is still in the order of magnitude of 15 amperes, the peak current, which is flowing for a sufficient length of time to cause the protective reactions to take place, will be typically as high as 300 amperes. The pulsed D.C. systems also cause a greater redistribution of the current along the structure, such as a pipeline, because of the inductive and capacitive reactance of the anode and structure system.
Copper-copper sulfate electrodes are conventionally used to determine the effectiveness of cathodic protection systems in protecting well casings and pipelines. Such electrodes, comprising a copper rod immersed in a copper sulfate solution (typically a gel) are placed in the ground, adjacent the well casings or pipeline (e.g., 1 or 2 feet there from) and the potential between the metal structure and the copper rod is measured. A potential, typically called "the well head potential", of about 1 volt is considered to provide appropriate protection.
Prior art cathodic protection systems are disclosed in my prior U.S. Pat. Nos. 3,612,898; 3,692,650; and 5,324,405 ("'405 patent"). The '405 patent teaches an improvement over the systems disclosed in the earlier patents in terms of increasing the current distribution or throw of the current along a pipeline or well casing as well as increasing the protection of neighboring pipelines or well casings. This improvement is accomplished by the limiting current flow in the power supply through the use of back emf current limiting means. The disclosure of the '405 patent is incorporated herein by reference.
A typical prior art pulsed protection system is illustrated in FIG. 1 of the drawings where reference numerals 10, 12 and 14 designate a D.C. voltage source, an anode/cathode voltage switch and a pulse width/frequency control unit, respectively. The positive output is supplied to an anode unit 16 (which may comprise several discrete metal cylinders connected in parallel) via a positive terminal 18 and the negative output is supplied to a plurality of well casings or pipelines 20 and 22 via the negative terminal 24. A diode 25 (or a back emf limiter as taught in the '405 patent) is connected across the output terminals 18 and 24. The voltage and current waveforms V and I of the output, appearing across the terminals 18 and 24, are shown in FIG. 1 to the right of the switch 12. As is pointed out in the '405 patent the use of diode 25 protects the voltage source from reverse voltage spikes at the expense of somewhat limiting the current throw and the protection for neighboring structures where a single current source is used.
A problem has arisen when a single pulsed D.C. source is used to protect two or more structures from a single anode unit where the spacial distances between the structures and/or the electrical properties of the soil or other conducting medium result in one or more structures receiving excessive current while others receive inadequate current for protective purposes. The use of a separate anode unit and pulsed sources for each neighboring well casing or pipeline has its own set of problems as is alluded to in the '405 patent. An under protected well casing or pipeline located in adverse soil conditions may need frequent replacement. The cost of replacing a damaged well casing or section of pipeline can be very expensive. For example, the cost to replace a deep well casing may run as much or more than one million dollars. Thus, the problem has serious economic consequences.
There is a need for an improved cathodic protection system capable of adequately protecting multiple adjacent structures such as well casings and the like which are not amenable to the use of a single pulsed source.